This invention relates to an industrial robot that scans a work point with a shape sensor before the point is worked.
There are many types of industrial robots. Some of them have a sensor for scanning certain reference points or lines of a workpiece. In one control system for such robots, a sensor in front of a tool senses the reference points and detects any shape or setting error of the workpiece. Interpolation values for the standard path are sequentially corrected during the profile control using the detected errors. This control system, however, has a timing problem when applying the sensed data from the sensor to the correction of the interpolation values. Specifically, relating one point T1, when the sensor detects a work point, and another point T2, when the tool actually arrives at the detection point, in real-time is difficult because of variable factors, such as the working speed and delays in the servo system of the robot.
To solve this problem, the following method has been proposed: work-shape data detected by the sensor are stored in a first-in/first-out buffer from which the oldest data are taken to calculate interpolation values for the tool following the sensor (Japanese Laid-Open Patent Publication No. 26276/1988). In this method, the time for starting the path correction can be adjusted according to the working speed if the working speed of the robot is equal to the speed at which the work-shape data stored in the buffer is used. Thus, it permits precise profile control for various working speeds of the robot as long as the speed is constant during one operation.
This control system, however, cannot always achieve the accurate profile control: if the robot changes the working speed during one operation, the relationship between the timing for using the work-shape data in the buffer for the path correction and the actual working point of the robot no longer applies The working speed of robot may change during an operation for the following reasons.
(1) Generally, robots change the working speed at the beginning or the end of the operation to prevent unnecessary oscillations.
(2) The working speed must be adapted for various local conditions such as flats and corners.
(3) Robots used for laser cutting move fast on the flat and straight parts of the workpiece and move slow on curves to reduce the total working period. If the working speed is fixed, the robot must move slowly throughout the entire piece. Moreover, since the robot never moves on only one line during any operation, several standard paths are typically taught based on several teaching points. Thus, the path of the robot necessarily changes.